It is well known that the heterogeneous exothermic processes can be carried out technologically in the fluidized reactors, the slurry phase reactors and in the fixed-bed reactors. The fixed-bed reactors are the most commonly encountered systems in the area of the catalytic technologies. Such reactor is a heterogeneous system that consists of at least two phases: solid particles of the catalyst and spaces in between where the reaction mixture flows in form of gas and/or liquid. Simultaneously the chemical conversions on the catalyst surface and the following physical processes take place in the reactor: the reaction component and product transfer in the bed, heat transfer and gas flow etc.
In the tubular reactor the heat transfer between the reaction mixture and cooling medium is realized through the reactor wall. The catalyst is packed into the tubes of the small diameter (2-8 cm); the cooling medium (e.g. high pressure steam) circulates in the intertubular space. The important advantage of the tubular reactors is favorable terms for heat removal from the catalyst because the ratio of the cooling surface to the catalyst volume is well over than in other apparatuses.
The formation of the optimal temperature range in the catalyst bed is one of the main problems faced by specialists during the development of the catalytic tubular reactors. The improvement of the heat and mass transfer in each catalyst pellet promotes it. It is possible to improve the heat transfer by using a support for catalyst based on the high heat conducting material and the mass transfer can be enhanced by using a support with isotropic properties.
US 20040024274 relates to a catalyst for the hydrogenation of a monocyclic or polycyclic aromatic hydrocarbons comprising at least one metal of transition group VIII of the Periodic Table and a structured or monolithic support. The structured supports have a regular two-dimensional or three-dimensional structure and they are in this way distinguished from particulate catalysts which are usually used in the fixed-bed reactor. The structured supports or monoliths can comprise metallic, inorganic, organic or synthetic materials or combinations of such materials. Examples of metallic materials are pure metals such as iron, copper, nickel, silver, aluminum and titanium or alloys such as steels, for instance nickel steel, chromium steel and molybdenum steel, brass, phosphor bronze, Monell and nickel silver. For example, the catalyst body can be made of metallic, structured supports or monoliths comprising stainless steel which preferably displays roughening of the surface when heated in air at 400-1100° C. during 1-20 hours and subsequently cooling to room temperature. Moreover the surface becomes enriched in an alloying constituent (for example, aluminum or chromium from which a corresponding surface layer of Al2O3 or Cr2O3 is formed). Roughening can also be carried out mechanically in place of or in addition to thermal roughening. Preference is given to using structured supports in the form of woven metal meshes or fabrics, knitted metal meshes or fabrics or metal felts, woven carbon fibers or carbon fiber felts or woven or knitted polymer fabrics or meshes.
Examples of inorganic materials are ceramic materials such as aluminum oxide, silicon dioxide, zirconium dioxide, cordierite and steatite. It is also possible to use carbon. Examples of synthetic support materials are, for example, polymers or glass fibers.
The catalysts may further comprise promoters, for example, alkali metals and/or alkaline earth metals (e.g. lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium); silicon, carbon, titanium, zirconium, tungsten and the lanthanides and actinides; coinage metals such as copper, silver and/or gold, zinc, tin, bismuth, antimony, molybdenum, tungsten and/or other promoters such as sulfur and/or selenium.
The structured supports or monoliths can be coated with one, two or more oxides by sputtering before applying the active components. The active metals are applied to the support by impregnation with aqueous or organic solutions or suspensions comprising the active components.
The hydrogenation is preferably carried out at from about 50 to 200° C. and at pressures of less than 50 bar. As a result the formation of undesirable by-products such as n-paraffins is virtually nonexistent, so that the purification becomes unnecessary; this makes the process very economical. When the process is carried out continuously, the amount of the compound to be hydrogenated is preferably from about 0.05 to about 3 kg/1 of catalyst per hour. Moreover the conversion of benzene and a yield of cyclohexane are 99.99%. The space-time yield was 0.928 kg/(l·h).
The disadvantage of such catalyst and the support is the following: the support nature does not allow obtaining the catalyst with high concentration of the active component in the reaction volume; it decreases the process productivity and results in increasing the reactor dimensions.
EP 1994982 relates to a catalyst for purification of exhaust gases, comprising an anodized layer (formed by subjecting the carcass-forming metal to anodizing treatment) having micropores thereon, and catalytically active metal in the micropores. The carcass-forming metal surface becomes covered by an oxide film during anodizing treatment; such metals include aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth and antimony, preferably aluminum, both pure aluminum and alloys. At least one element oxide (able to adsorb NOx) selected from the group consisting of K, Ba, La, Ca, Li, Ce, Zr, Pr, Nd, Te, Sm and Gd in an amount of at least 0.01 g/m2/mm and at least one element selected from the group consisting of Pd, Pt, Rh, Ru, Ir, Ag and Ga in an amount of at least 0.15 g/m2/μm are used as catalytically active metal. The catalyst further comprises the porous material having the ability to adsorb ammonia; the material is applied on the anodized layer. The anodized layer is obtained as follows: firstly the support is subjected to treatment for the grained surface formation then anodizing treatment takes place. The porous material is applied on the anodized aluminum layer as a mixture containing a zeolite in combination with at least one of phosphoric acid and/or colloidal silica and/or boehmite sol. The amount of the porous material is defined based on the required thickness of the anodized layer and its porosity. The catalyst body surface has a micropore density of from 50 to 10000 pores/μm2, and the micropores preferably occupy from 20 to 80% of the surface area.
Such catalyst comprises small amount of precious metal and it is effective for purification of exhaust gases of diesel engines from nitrogen oxide (NOx) even at relatively low temperatures (150-250° C.).
The disadvantage of the catalyst and support is great difficulty of the method of obtaining. The method comprises the following stages:
1) heat treatment of the carcass-forming metal;
2) grained surface formation;
3) anodizing treatment;
4) acid or alkali treatment of the anodized layer;
5) hydration treatment;
6) application of oxide able to adsorb NOx;
7) application of active metal;
8) application of porous material able to adsorb ammonia.
Such multistage method makes difficult the catalyst production on an industrial scale.
EP 2033709 provides a catalyst containing the support and catalytic component. The support is a metal substrate with aluminum-containing surface; the metal substrate is treated for formation rough layer on its surface. The metal substrate is a metal fiber, metal foil, metal mesh or metal plate. The rough surface of the substrate contains aluminum, the aluminum surface is scaled (flaky). The metal substrate is subjected to ultrasound or microwaves by immersing in a solution with hydrophilic alcohol or hydrocarbons, and then the substrate is calcinated at 300-500° C., during 0.5-1.0 hour. The period of the ultrasound or microwaves treatment is preferably set within a range that an amount of precipitation of iron or chromium is less than that of aluminum. The precious metals, such as platinum, palladium, and rhodium are used as catalytic component; oxides can be used as support. The support contains cerium, cerium, zirconium, alkali metals, alkaline-earth metals and oxides, for example, alumina. The catalyst can be used in the process of purification of exhaust gases. CO conversion is 83-99% and it depends on the catalyst layer thickness.
All cited catalysts have structured or monolith supports, their preparation for applying catalytic material is quite difficult. This fact limits the practical use of such catalysts.
The catalysts having powder of aluminium (consisting of particles in form of flakes or scales) or pellets containing aluminium in form of flakes as support component do not have such disadvantages. Moreover the metallic aluminium has high heat conductivity and improves the heat transfer from the reaction zone. The facts increase the stability of the catalyst function.
RU2326732 provides the catalyst for Fischer-Tropsch synthesis comprising a metal of the VIII group of the Mendeleev's Periodic Table of the Elements as active component, a support containing an oxide component and the metallic aluminium in the form of flakes. The active component content is 5-40% based on the total weight of the catalyst. The oxide component contains aluminium oxide and/or silicon oxide and/or titanium oxide and/or zirconium oxide. The content of aluminium in the form of flakes is 1-25% based on the total weight of the catalyst. Metals of the VII-VIII groups of the Mendeleev's Periodic Table of the Elements and/or oxides can be used as promoters.
The support is obtained by extrusion of a paste, followed by the heat treatment and disintegration to the fraction with desired size. The paste comprises an oxide component, metallic aluminium in the form of flakes, diethyl ether, water and a binder. 5-15% wt boehmite based on the total weight of the catalyst can be used as the binder. The active component and promoters are applied by impregnation, followed by drying, calcinating on air flow.
Before carrying out the synthesis, the catalyst is activated by reduction in the hydrogen stream (GHSV 100-5000 l/h) at a temperature in the range from 300 to 600° C. during a time period from 0.5 to 5 hours. Synthesis of hydrocarbons from CO:H2 is carried out in a tubular reactor with a fixed bed of the catalyst under a pressure in the range from 0.1 to 4 MPa, and a temperature in the range from 150 to 300° C. The molar ratio of CO:H2 in synthesis gas is in the range from 1:1 to 1:3. The synthesis gas can contain up to 25% nitrogen.
The catalyst gives CO conversion of 48-83%, methane selectivity of 6-12%, C5+ selectivity of 75-90% and productivity of 85-112 kgC5+/m3/h.
The disadvantage of the catalyst and the support is much hampered mass transfer in the radial direction of the pellet; the reason of the facts is in obtaining of the support from the paste containing aluminium in the form of flakes, which get easily aligned during extrusion. Therefore the support has strongly marked anisotropy of the properties, i.e. has different properties in the directions perpendicular and parallel to the direction of the movement of the extruded body.
As well as use of aluminium in the form of flakes gives an increased risk because of the pyrophoricity of the flakes, hence it makes difficult the obtaining of the support and the catalyst. Moreover aluminium content is insufficient in order to provide the desired heat removal from the active centers when the synthesis gas GHSV in the catalyst bed is more than 3000 l/h. This is due to that the main part of the active component is deposited on the surface of aluminium; the fact decreases the efficiency of using the active component.
RU2256501 relates to a catalyst and support for synthesis of hydrocarbons from CO and H2 based on cobalt. The catalyst comprises the powder of metallic aluminium as support. The catalyst can comprise promoter selected from the group of the metal oxides, e.g. ZrO2, La2O3 or K2O or metals, e.g. Re, Ru, Pd or Pt.
The catalyst is prepared as follows: the aluminium powder (in accordance with GOST (State Standard, Russian Federation) 5494-95 powder consisting of particles of scaled or flaky form) is impregnated with an aqueous solution of cobalt nitrate. Then the impregnated powder is dried on water bath and calcined at 450° C. for 1 hour on air flow. The catalyst is activated by reduction in the hydrogen stream (GHSV 100-3000 l/h) at a temperature in the range from 400 to 600° C. during a time period from 1 to 5 hours. Synthesis of hydrocarbons from the mixture of 66-68 mol. % H2 and 32-34 mol. % CO is carried out under the atmospheric pressure and a temperature in the range from 160 to 230° C. with syngas GHSV of 100 l/h.
In spite of improved diffuse mass transfer in comparison with ordinary pellets, the disadvantage of the catalyst is low activity and productivity of the synthesis because of low synthesis gas GHSV in the catalyst bed and impossibility to increase it.
As well as it is impossible to use the catalyst in the industrial fixed bed reactors because the support and the catalyst on the support are in the powdered (pulverized) form. The fact increases considerably the pressure drop in the reactor to the value that is inadmissible for the process performance.
Therefore the metallic aluminium in the known supports and the catalysts is used in the form of powder from the flakes or the morphology of the particles is not disclosed.
However use of aluminium in the form of flakes causes the troubles of the mass transfer in the radial direction, it defined by the strongly marked anisotropy of the properties.
It should to be noted that use of aluminium powder in flakes is characterized by the increased danger because of the large open space, this fact results in difficulties for the support preparation.
In accordance with GOST 5494-95 the powders of aluminium in the form of flakes or “aluminium powders” consist of the particles of the flaky (scaled) form. They are manufactured by grinding of the primary aluminium of grade at least A5 in accordance with GOST 11069 or from the aluminium waste (the chemical composition of the waste is the same as aluminium of the grade A5). Particularly, the largest producer RUSAL manufactures the powder of five following grades: PAP-1, PAP-2, PAG-1, PAG-2, PAG-3. The scaled aluminium powders are used for manufacture of the “silvery” paints and in the defense industry.
As well as the powders of the “spherical” aluminium in particles of globular form are well known in the pyrotechnics, the ferrous metallurgy, the chemical industry, the welding engineering. The powders are produced by pulverization of molten aluminium in an inert gas heated to the melting temperature with simultaneous supplying of cold inert gas into the zone of atomization (see RU2081733).
Particularly, the largest producer RUSAL manufactures the powders of the “spherical” aluminium of grade ASD according to the standard 48-5-226-87. These powders are defined by less specific open surface and hence their use is less dangerous.